This invention relates to displays or detectors and to optoelectronic devices suitable for use therein. In particular it relates to semiconductor optoelectronic elements and to the manufacture thereof, and to a semiconductor color display/detector comprising a plurality of such optoelectronic elements. The invention also concerns an alloy material for use in the fabrication of such optoelectronic elements, color displays and detectors.
Known color displays can be split into two main types: i) vacuum or valve displays such as the cathode ray tube or gas discharge lamp and ii) thin, flat screen displays such as liquid crystal or electroluminescent displays. The foregoing display types have a number of drawbacks associated with their manufacture and use. Whilst the manufacture and mass production of CRTs and gas discharge lamps is a well established and reliable technology, these display devices are bulky and relatively fragile. Additionally, CRTs, gas discharge tubes and the like consume large amounts of power and are therefore unsuitable for portable or mobile applications. The aforementioned flat displays overcome some of these problems in that they are generally more robust, consume less power and are less bulky than CRTs and the like. They are also susceptible to mass production since similar techniques as are used in integrated circuit production can be applied to their manufacture. However, their manufacture is somewhat more complicated than IC production, since the optically active material has to be sandwiched between two substrates. In addition, the substrates support the electrodes necessary to stimulate the optically active material and their associated signal lines, thereby resulting in a complex multilayer structure. Drawbacks of known flat displays are that they have poor color uniformity, low resolution, viewing angle and brightness, and are slow to respond to updated information in comparison to CRTs and the like.
Known color detectors can be split up into three main types:
(i) vacuum or valve detectors such as Orthicon or Vidicon .TM. tubes;
(ii) thin, flat detectors such as photo diodes optimised or doped to detect a particular wavelength, or band of wavelengths, of light; and
(iii) thin, flat detectors such as Charge Coupled Devices (CCD) utilising color filters.
The foregoing color detectors have a number of drawbacks associated with their manufacture and use, particularly when used as part of full color detectors.
In the case of Vidicon .TM. tubes, a full color detector comprises either three separate tubes, each tube having one primary color input thereto, or a single tube having a special target system in the form of a matrix having color-discriminating properties. Light incident on a full color detector utilising a Vidicon .TM. tube or tubes, is split into its primary color components by optical filters. Consequently the detector is bulky and heavy due to the electrical components required to produce and control the scanning electron beam and the optical components required in the detector. It is also sensitive to vibrations causing misalignment of the electron beam and the optical components.
Photo-diodes are unsuitable for use in a full color detector because a triad of three photo-diodes, one of each optimised for a different primary color, is necessary to form a single color picture cell. Because the constituent materials, doping materials or amount of doping are different for photo-diodes optimised for different colors it is very difficult to fabricate triads where each photo-diode is close enough to its neighbours to appear to be part of the same picture cell. For this reason, photo-diodes and the like are unsuitable for color camera applications.
Full color detectors utilising CCD's have a number of advantages over vacuum or valve devices, because they are of smaller size and have greater physical and optical ruggedness. Nevertheless, CCD full color detectors still require some form of color filtering in either the multi-tube or single tube format. Again this leads to undesirable bulkiness and weight. A further disadvantage of CCD full color detectors, is that when they are used as the target in color cameras they exhibit inadequate definition and have poor resolution.
Many of the drawbacks associated with known color displays and detectors are due to the fact that hitherto optoelectronic elements capable of being adapted to operate in the visible red to visible blue (and also U-V) region of the spectrum and made from substantially the same materials, were unknown. This resulted in optoelectronic devices being built on different substrates and having to have complicated and bulky interconnections therebetween. Furthermore, known optoelectronic devices were not fabricated on silicon substrates. As a result, well known silicon VLSI technology techniques could not be monolithically incorporated into the optoelectronic devices.
In particular, optoelectronic devices comprising semiconductor material having band-gaps which are wide enough to emit/absorb blue light have proven very difficult to grow and dope in a controlled and systematic manner. II-VI compound semiconductors whilst having suitable band-energies for blue light, are not suitable for depositing on a silicon substrate and invariably have to operate at low temperatures. Wide band-gap nitride semiconductors have been studied as alternatives to II-VI compounds with varying degrees of success. Although blue light has been emitted from GaN devices, such devices had to be built on sapphire substrates, which is prohibitively expensive for commercial applications, and the quality of the resulting material was poor due to lattice mismatch with the substrate and the introduction of nitrogen vacancies due to the relatively high growth temperatures.
Examples of certain known solid state color detection devices which do not use color filters, and which include arrays of detection devices sensitive to different color portions of the electromagnetic spectrum are disclosed in EP-A-0166787 and U.S. Pat. No. 5,138,416. In EP-A-0166787, the devices are electrostatic induction phototransistors (SIT), which operate on the principle that the gate potential of the SIT is determined by light directed thereto. Structural dimensions of the SIT's, such as diffusion depths of the SIT gate regions, are varied to provide the required selectivity of spectral sensitivity. Blue light response is improved by limiting the depletion layer thickness.
In U.S. Pat. No. 5,138,416, a multilayer color photosensitive element is made from group III-V alloy semiconductors. Charge is collected from the photosensitive element according to an amount of time it takes for red, green and blue light photoexcited carriers to travel through the layers. The resulting waveform obtained from the element will show temporal features which depend on the colors of illumination.
Examples of certain known solid state color display devices including electroluminescent elements emitting different colors are disclosed in U.S. Pat. No. 3,890,170 and U.S. Pat. No. 4,211,586. In U.S. Pat. No. 3,890,170, two matrices of respectively, red LED's and green LED's are integrated and can provide a red, green or orange display. The red LED's are gallium arsenide phosphide and the green LED's are gallium phosphide, and the total LED array is formed on a monolithic semiconductor substrate of gallium arsenide or phosphide. Row and column address lines are provided on the substrate for each matrix so that a strobing logic address system can effect lighting of the individual LED's for producing an alphanumeric character or graphic display.
In U.S. Pat. No. 4,211,586, an array of red, green, orange and yellow light emitting diodes is formed on a common gallium arsenide substrate. A vertically graded epitaxial region of GaAs.sub.1-x P.sub.x is selectively etched to different depths, and diodes of different color emission are formed at selected concentration levels in the epitaxial region.
In neither of these color display devices is full color display possible, as the materials employed are not capable of blue light emission.
Specific LED constructions for blue light emission are disclosed in GB-A-2250635 and U.S. Pat. No. 5,005,057. In GB-A-2250635 a luminescent layer epitaxially grown on a buffer layer, in turn formed by epitaxy on a semiconductor substrate of zinc sulphide, zinc selenide or a mixed crystal thereof, is formed from aluminium nitride, indium nitride, gallium nitride or a mixed crystal of at least two of such nitrides.
In U.S. Pat. No. 5,005,057, a blue LED has either a superlattice structure of alternately stacked BP and Ga.sub.x Al.sub.1-x N (0.ltoreq.x.ltoreq.1) layers, or a mixed crystal structure of Ga.sub.x Al.sub.y B.sub.1-x-y N.sub.z P.sub.1-z (0.ltoreq.x,y,z.ltoreq.1 and x+y.ltoreq.1). Single and double heterojunction structures are disclosed.
The present invention aims to alleviate the earlier-mentioned disadvantages of known color displays and to provide a color display that is thin, lightweight, robust, simple to manufacture, consumes low power and yet has high levels of brightness and resolution and a wide range of viewing angles.
The present invention also aims to alleviate the earlier-mentioned disadvantages of known color detectors and provide a color detector that is lightweight, robust, simple to manufacture, has a simplified construction and has a high level of resolution.
The present invention is based on the recognition that certain group III-V nitride alloys can, by suitable choice of elemental composition ratios, be fabricated so as to be substantially lattice matched to a commonly used material substrate (Si or GaP) and so as to form the basis of an optoelectronic element which can by appropriate doping or quantum confinement techniques be made to emit/absorb light at any desired wavelength in the visible red to ultra-violet region of the electromagnetic spectrum, including blue.
In one aspect, the present invention provides a color semiconductor display/detector device having a plurality of optoelectronic elements, said plurality including a first element and a second element, said first and second elements being capable of emitting/absorbing light having respective different predetermined wavelengths in the range corresponding to visible light, wherein said first and second optoelectronic elements are formed on a common substrate, and comprise III-V nitride alloy compositions, said compositions being substantially lattice matched to said common substrate.
In another aspect, the invention provides an optoelectronic element suitable for use in such a display/detector having an active region comprising InNSb or GaAsN. Preferably the active region comprises a quantum well region of InNSb or GaAsN and a barrier region of AlNSb.
A particular advantage of a display/detector according to the present invention is that it can be manufactured in a "one step" epitaxial growth process without the need to assemble separate components and inject optically active material between substrates.
In a preferred embodiment of the invention the substrate comprises either Silicon (Si) or Gallium Phosphide (GAP), and the III-V nitride alloy includes Sb. The alloy preferably comprises either InAlNSb or AlGaAsSbN.
In particular, the device of the present invention comprises a plurality of active quantum well regions confined by barriers consisting substantially of AlN.sub.1-x Sb.sub.x, where x lies in the range 0.57.ltoreq.x.ltoreq.0.63 and having a quantum well region comprising substantially InN.sub.1-y Sb.sub.y where y lies in the range 0.32.ltoreq.y.ltoreq.0.38 for optoelectronic elements of InAlNSb alloy, and comprising substantially GaAs.sub.1-z N.sub.z where z lies in the range of 0.17.ltoreq.z.ltoreq.0.23 for optoelectronic elements of AlGaAsSbN alloy.
In a further aspect of the invention there is provided an optoelectronic element comprising a III-V nitride alloy capable of being substantially lattice matched to Si or GaP, and of being selectively adapted to emit/absorb light in the visible red to ultraviolet region of the spectrum, the element having an active region comprising InNSb. The active region preferably comprises an active quantum well region of InN.sub.1-y Sb.sub.y, where y lies in the range 0.32.ltoreq.y.ltoreq.0.38 and barrier regions of AlN.sub.1-x Sb.sub.x where x lies in the range 0.57.ltoreq.x.ltoreq.0.63.
In another aspect of the invention there is provided an optoelectronic element comprising a III-V nitride alloy capable of being substantially lattice matched to Si or GaP, and of being selectively adapted to emit/absorb light at a desired wavelength in the visible red to ultra violet region of the electromagnetic spectrum, said element having an active region comprising an active quantum well region of GaAsN and barrier regions of AlNSb. Preferably, the active quantum well region comprises GaAs.sub.z-1 N.sub.z where z lies in the range 0.17.ltoreq.z.ltoreq.0.23, and barrier regions of AlN.sub.1-x Sb.sub.x where x lies in the range 0.57.ltoreq.x.ltoreq.0.63.
In a yet further aspect of the invention, there is provided a nitride alloy comprising InN.sub.1-y Sb.sub.y where y lies in the range 0.32.ltoreq.y.ltoreq.0.38, and a nitride alloy comprising AlN.sub.1-x Sb.sub.x where x lies in the range 0.57.ltoreq.x.ltoreq.0.63.